Measurement tools are in widespread use in borehole formation testing, for example in boreholes drilled into the earth in order to test for or recover underground reserves of oil and/or gas. Some such tools are carried by the drill string and the measurements are carried out during the borehole drilling operation (so-called “measurement-while-drilling” (MWD) or “logging-while-drilling” (LWD) applications). Other measurement tools are used after the borehole has been drilled, the measurement tools being lowered into the borehole by a cable or wire. In highly deviated wells conveyance may be assisted by semi-rigid tubing or by drill-pipe. Still other measurement tools are deployed downhole for lengthy periods of time with or without a connecting cable and are referred to as permanent or retrievable gauges. These are usually for use in production after the exploration phase is complete.
Tools deployed using cable having one or more electrical conductors are generally referred to as “electric wireline tools”. The present invention is most likely to be a part of an electric wireline tool, though its use in MWD/LWD or other downhole applications is not thereby excluded.
One known electric wireline tool is a formation testing tool or “pump-out” tool, which is used to extract a volume of fluid from a formation surrounding a borehole, the fluid being tested in order to evaluate the likely productivity of the oil or gas well.
It is a recognised problem of operating formation testing tools that during the borehole drilling operation the fluid within the formation can be contaminated with drilling fluid (or “mud”) filtrate typically comprising liquid and other materials. In order to obtain valuable test results it is of prime importance that the formation fluid used for analysis represents virgin formation fluid with little or no contamination from fluids used in the borehole drilling operation.
Drilling fluid is generally divided into oil base mud (OBM) and water base mud (WBM). The drilling fluid pressure is maintained higher than that of the formation, and as a result the drilling fluid seeps into the formation, the seeping fluid being known as filtrate. Fine particles that cannot penetrate the formation are left behind on the borehole wall and build up to form a filter (or “mud”) cake. This is relatively impermeable and forms a skin substantially preventing further ingress of fluid. The filtrate displaces virgin formation fluid from the vicinity of the borehole wall, until a stable ‘invaded zone’ results. Depending on the virgin fluid, the type of mud and the formation composition and structure, different degrees and depth of invasion occur into the formation.
The formation fluid may naturally contain a large percentage of water, of some salinity. Water base mud is predominantly water but need not have the same salinity. Although perfect oil base mud has very little water, in practice it may contain as much as 40% water. Filtrate may include formation water from other depths in the borehole that has mixed into the mud.